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BISMARCK — Oil from a major pipeline leak near Tioga last fall has seeped at least 30 feet below ground, “and we think it’s gone down farther than that,” the North Dakota Department of Health’s environmental section chief told state lawmakers Tuesday.

Dave Glatt briefed the Legislature’s interim Energy Development and Transmission Committee on the progress of cleaning up the spill that was discovered by a landowner in his wheat field near Tioga on Sept. 29.

Tesoro Logistics has recovered more than 6,000 barrels of the estimated 20,600 barrels of oil that leaked from its pipeline, Glatt said. The company initially estimated the spill at 750 barrels and was allowed to burn oil off the surface before it released the revised estimate on Oct. 8.

Glatt said the company built cutoff walls to stop the oil from spreading farther and drainage systems to collect the oil. Tesoro also conducted tests to determine the area of contamination.

Glatt said there is “a lot of cushion” between the contamination that extends downward at least 30 feet and the groundwater that sits at about 150 feet below ground.

“But we need more refined data to get that footprint, to know the surface area and how far down it’s going,” he said.

Tesoro plans to dig as deep as 50 feet to excavate as much contaminated dirt as possible, Glatt said.

“This is going to be a big hole,” he said.

Tesoro’s remediation plan calls for cooking the oil out of the soil and returning the soil to the site, a process expected to take about two years, Glatt said..

“It’s a long process, but at the end of the day I believe we’ll get the vast majority of the contamination,” he said.

A preliminary report from the federal Pipeline and Hazardous Materials Safety Administration indicated a strong electrical discharge likely caused the quarter-inch-diameter hole in the pipeline, but a final report has yet to be issued.

Lawmakers also were updated Tuesday on the cleanup of more than 400,000 gallons of oil that spilled Dec. 30 near Casselton when a BNSF train carrying soybeans derailed and was struck by an oncoming train hauling crude oil.

Glatt said BNSF has excavated about 9,000 yards of material, disposing of about 1,400 yards in North Dakota landfills and the rest of it out of state. The state will require BNSF to contain any runoff in the springtime and to conduct further testing to make sure all of the oil has been recovered and isn’t leaching downward, he said.

The railroad removed the top 2 feet of soil and will replace it with fresh soil, he said.

“But we want them to go a little bit deeper to make sure that they’ve got everything there,” he said.

Glatt said BNSF hopes to complete the remediation by this summer.

State Sen. Connie Triplett, D-Grand Forks, said the state can’t keep looking to the federal government to provide more inspectors and resources to prevent such oil-related incidents.

“I think at some level we have to say, if we’re going to promote this industry and encourage this industry, we have to also accept some responsibility for making sure that the public … is safe. So, I think we need to ask that of all of our agencies, to tell us how we can do it better,” she said.

Senate Majority Leader Rich Wardner, R-Dickinson, the committee’s chairman, agreed that the state must do more, voicing support for an idea being discussed by the Public Service Commission to create a state-run oversight program for intrastate oil pipelines.

“But it really bothers me … when the federal government has resources in other areas to dig around in our lives,” Wardner added.